1. Field of the Invention
The present invention relates to a focus jump device for jumping between recording layers in a storage medium, such as an optical disc, having at least two recording layers.
2. Description of Related Art
In a conventional optical disc reproduction apparatus such as a Compact Disc player, a focus servo control is performed as follows. First, a predetermined drive signal of a sawtooth waveform, for example, is supplied to a drive circuit of an actuator, and then the drive circuit applies an output corresponding to the drive signal to a focus coil provided in the actuator. Thus, the actuator moves toward or away from the disc. Then, a laser beam is irradiated on the optical disc by the objective lens of the actuator, and the light reflected by the optical disc is converted into an electric signal by the photodetector to produce a focus error signal. The focus error signal has a zero-level output when the relative distance between the objective lens and the disc is equal to a reference value, and has an S-shaped characteristic in which the output level continuously varies dependently upon the displacement from the reference distance. The focus error signal thus produced is compared with the reference value, and the comparison result is inputted to the servo loop control unit as a positional error. The servo loop control unit recognizes the zero-crossing point of the focus error signal from the inputted positional error, and inputs the focus error signal to the drive circuit of the actuator when the focus sum signal, indicating the total quantity of the reflected light, is larger than a given level and the focus error signal is at the zero-crossing point. Thus, the focus servo loop is established by the actuator, the photodetector and the drive circuit of the actuator. When the focus servo loop is established, the focus error signal is subjected to the phase compensation by the equalizer amplifier, and then inputted to the drive circuit of the actuator. The drive circuit of the actuator outputs the drive signal to the actuator, based on the inputted focus error signal, to drive the objective lens such that the relative distance between the objective lens and the optical disc is always equal to the reference value. In this way, in a conventional CD player, the pickup is driven by the focus coil to constantly maintain the relative distance between the objective lens and the optical disc to be constant.
Recently, a disc having a multi-layer structure including a first reflective layer and a second reflective layer to achieve high density recording has been put into practice along with the increase of the recording information on an optical disc. In reproducing such an optical disc having multi-layer structure, the focus servo control should be reliably performed for the reproduction layer from which recorded information is to be read out, when the reproduction layer is changed from one to another. For example, in order to change the reproduction layer from the first reproduction layer to the second reproduction layer during the reproduction, the actuator should be jumped to move the focused beam spot from the first reproduction layer to the second reproduction layer. The interlayer jump of the beam spot like this is generally called as xe2x80x9cfocus jumpxe2x80x9d. Specifically, the focus jump is performed as follows. In response to the jump start command, an acceleration pulse of a constant amount is applied to the drive circuit of the actuator, and a deceleration pulse of a constant amount is then applied to the drive circuit at a desired timing. By this, an open control of feed-forward type is carried out, and then the above described focus servo control is resumed after the objective lens reaches the target position. The target value of the focus servo is zero, and the detection system shows a linear characteristic when the focus error stays around zero at the focus servo closing timing. However, the output of the detector shows non-linear characteristic such as a sinusoidal waveform when the interlayer jump (i.e., focus jump) is performed. Therefore, conventionally, the feedback control for stabilization is not performed during the, focus jump period.
However, the open control of the feed-forward type is susceptible to external disturbance. For example, the control may be unstable due to the vibration of the disc surface at the time of the focus jump, or variation of the interlayer spacing. Further, the servo may not be quickly set at the time of closing the focus servo after the focus jump, or the focus servo may lose control, thus leading to the deterioration of performance.
It is an object of the present invention to provide a focus jump device which performs stable focus jump even if external disturbance such as the vibration of the disc surface or variation of the interlayer spacing exists.
According to one aspect of the present invention, there is provided a focus jump device, adapted to be used in an information reproduction apparatus for reproducing information from a storage medium having at least two recording layers or used in an information recording apparatus for recording information onto the storage medium, which moves a focus position of a light beam in a direction perpendicular to the storage medium to the focus position corresponding to a target recording layer so that the light beam is irradiated on the target recording layer, the device including: a focus position moving unit for moving the focus position of the light beam; a focus error signal detector for detecting a focus error signal based on a variation of a returning light from the storage medium, the variation of the returning light being based on a variation of the focus position of the light beam with respect to the recording layer; a feedforward controller for supplying a drive signal to the focus position moving unit to move the focus position of the light beam in an acceleration state in an acceleration control and to move the focus position of the light beam in a deceleration state in a deceleration control; and a position controller for comparing the focus error signal, which is detected by the focus error signal detector at the time of movement of the focus position of the light beam to the target recording layer triggered by the acceleration control, with a predetermined target value to generate an error, and for feeding back the drive signal to the focus position moving unit to reduce the error thereby to control the focus position of the light beam.
In accordance with the focus jump device thus configured, when the jump instruction is outputted by an external controller or else, the feedforward controller conducts on the focus position moving unit. By this, the focus position of the light beam gradually starts moving, and the relative distance between the focus position of the light beam and the recording layer varies. In addition, by this variation, the returning light from the storage medium also varies, and this variation is detected by the focus error signal detector as the focus error signal. The focus error signal thus detected is compared with a predetermined target value to obtain an error therebetween, and a drive signal is fed back to the focus position moving unit such that the position controller reduces the error, thereby to perform the focus position control of the light beam. On the other hand, after the acceleration control, the feedforward controller performs the deceleration control at the given timing so that the focus position of the light beam reaches the target position of the recording layer. Thus, according to the present invention, with the aid of the feedforward control, the focus position of the light beam is securely moved according to the movement distance between the recording layers, which is longer than the movement distance in the focus servo control. In addition, the focus error signal, serving as a position information corresponding to the movement, is controlled to follow the given target value by the feedback control, and hence the focus jump is resistant to the external disturbance. As a result, the convergence of the focus servo after the focus jump is accelerated.
The feedforward controller may perform a constant speed control for a predetermined time period between the acceleration control and the deceleration control. If the feedback control is performed by the position controller during the constant speed period, the apparent frequency band is lowered, and stable servo loop is established. As a result, the focus position of the light beam can follow the given target value with high accuracy. When the feedforward controller performs the deceleration control after the constant speed period, the focus position of the light beam accurately reaches the target recording layer. Thus, highly accurate focus jump can be performed and the focus servo can rapidly converge after the focus jump.
The time period of the constant speed control may be set to be longer than the time periods of the acceleration control and the deceleration control. By this, the feedback control for the focus position of the light beam is effectively performed, and hence highly accurate focus jump can be achieved.
In a preferred embodiment, the feedforward controller may perform a constant speed control in which no pulse is applied to the focus position moving unit after the acceleration control in which at least one acceleration pulse is applied to the focus position moving unit, and may perform the deceleration control in which at least one deceleration pulse is applied to the focus position moving unit after a completion of the constant speed control. By this, since a sufficient constant speed period is provided between the initial acceleration pulse and the last deceleration pulse, the feedback function for the focus position of the light beam can be effective, thereby achieving the accurate focus jump.
In another preferred embodiment, the feedforward controller may perform the deceleration control in which a plurality of deceleration pulses are applied to the focus position moving unit stepwise after the acceleration control in which at least one acceleration pulse is applied to the focus position moving unit, and may perform a constant speed control in which no pulse is applied to the focus position moving unit in a time period between the acceleration pulse and the deceleration pulse and time periods, between the deceleration pulses. Thus, the focus position of the light beam moves to the position close to the target recording layer at first, and then accurately reaches the target position with gradually decelerating. As a result, the highly accurate control can be performed and the focus servo can be quickly converged after the focus jump.
In still another preferred embodiment, the feedforward controller may perform the deceleration control in which at least one deceleration pulse is applied to the focus position moving unit after the acceleration control in which a plurality of acceleration pulses are applied to the focus position moving unit stepwise, and may perform a constant speed control in which no pulse is applied to the focus position moving unit in time periods between the acceleration pulses and in a time period between the acceleration pulse and the deceleration pulse. Thus, the focus position of the light beam gradually starts moving. Therefore, the influence by the surface vibration of the storage medium can be securely removed, and accurate focus jump can be achieved. In addition, the control is effective from the start of the focus jump, and the variation of the initial condition of the focus jump can be suppressed.
In still another preferred embodiment, the feedforward controller may include a plurality of application patterns of acceleration pulse and deceleration pulse for the acceleration control and the deceleration control, and may select the application pattern in accordance with a jumping distance. Since the application pattern of the pulse is selected according to the focus jump distance, the stable focus jump can be achieved regardless of the jump distance, and the required jumping time period can be shortened.
The feedforward controller may include a profile setting unit for setting a profile indicating a variation of the focus position of the light beam according to a passage of time, and a feedforward compensator may have a transfer function reverse to a transfer function of the focus position moving unit. The profile setting unit may set a profile including at least one focus position variation in a constant speed state between the focus position variation in the acceleration state from a start of a focus position movement and the focus position variation in the deceleration state up to an end of the focus position movement, and the feedforward compensator may apply a feedforward signal, which compensates for the profile, to the focus position moving unit. Therefore, the feedback control is performed in parallel with the feedforward control thereby to achieve accurate focus jump. In addition, the profile can be readily changed to freely control the feature of the position variation of the focus position.
The device may further include a linearizing converter for linearizing the focus error signal, and a reference position setting unit for setting a reference position indicating the focus position variation according to a passage of time. The feedforward controller may compare an output value of the linearizing converter with the reference position to an error therebetween, and may supply the drive signal to the focus position moving unit to reduce the error. Thus, the apparent frequency band in the feedback control is lowered. Hence, the focus position reaches the target position with high accuracy, and the focus servo rapidly converges after the completion of the focus jump.
The device may further include a linearizing converter for linearizing the focus error signal, wherein the feedforward controller uses the profile set by the profile setting unit as a reference position serving as the predetermined target value, compares an output value of the linearizing converter with the reference position to obtain an error therebetween, and supplies the drive signal to the focus position moving unit to reduce the error. Therefore, the apparent frequency band in the feedback control is lowered, and the focus position of the light beam follows the reference position. Thus, the focus position of the light beam can accurately reach the target position on the recording layer, and the focus servo can rapidly converge. In addition, the overall configuration can be simplified by the common use of the reference position setting unit and the profile setting unit.
The device may further include an area discriminator for discriminating whether the focus error signal detected by the focus error signal detector belongs to a first area in which the focus error signal is increasing or a second area in which the focus error signal is decreasing, wherein the linearizing converter includes a plurality of different conversion tables for different areas and selects the conversion table based on a discrimination result by the area discriminator thereby to linearize the focus error signal. Thus, the non-linear focus error signal is appropriately linearized. As a result, suitable feedback control can be achieved, the focus position of the light beam reaches the target position with high accuracy, and the focus servo can rapidly converge after the focus jump.
The device may further include a detection method discriminator for discriminating a detection method of the focus error signal, wherein the linearizing converter includes a plurality of different conversion tables for different areas for different detection methods, and wherein the linearizing converter selects the plurality of different conversion tables based on a discrimination result by the detection method discriminator and then selects the conversion table based on a discrimination result by the area discriminator thereby to linearize the focus error signal. Therefore, the linearization of the focus error signal is appropriately performed even if focus error signals of different waveforms are obtained, and appropriate feedforward and feedback controls are performed to achieve accurate focus jump.
The area discriminator may discriminate whether the focus error signal detected by the focus error signal detector belongs to a first area in which the focus error signal is an increasing positive value or a decreasing negative value, or a second area in which the focus error signal is increasing or decreasing, or a third area in which the focus error signal is increasing negative value or a decreasing positive value. Thus, non-linear focus error signal can be appropriately linearized. In addition, the feedforward and feedback controls are performed to achieve accurate focus jump.
In a preferred embodiment, the area discriminator may discriminate the area based on a timing at which the focus error signal reaches a positive or a negative peak value. Thus, the areas are accurately discriminated and the linearization is performed by using the appropriate conversion table.
The area discriminator may discriminate the area based on a quantity of the light beam returning from the storage medium. Therefore, the linearization is performed by using the appropriate conversion table.
The reference position may include a focus position variation at least one focus position variation in the constant speed state between an acceleration state from a start of the focus position movement and a deceleration state up to an end of the focus position movement. Therefore, the frequency band in the feedback control is lowered, and accurate focus jump can be performed with the stable feedback loop.
If there is a constant time period in which the focus error signal or a quantity of the light beam returning from the storage medium is smaller than a predetermined threshold and is substantially constant, the feedforward controller may perform the feedforward control in a time period other than the constant time period. Therefore, highly accurate feedback control is performed to achieve accurate focus jump.
The device may further include a focus servo controller for performing the focus position control of the light beam by feeding back a drive signal to the focus position moving unit such that the focus error signal becomes zero after the focus jump is finished, and each of the feedforward controller and the focus servo controller includes a phase compensator corresponding to its own control characteristic. Therefore, the appropriate controls are performed in different frequency bands.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below.